Mobile wireless channel environment includes a multi-path characteristic between a base station and a mobile station due to radio wave obstacles, which are distributed all around and have various sizes and materials, and also has a time-variable characteristic of a received signal since the mobile station or radio wave obstacles are moved.
A multi-path between the base station and the mobile station has paths of different lengths, and has a delay spread characteristic in which a received signal is lengthily extended. A wireless packet channel has time selective fading where a channel is varied according to time due to this characteristic and frequency selective fading having frequency components of different sizes and phases due to delay reception through multiple paths having a variety of lengths, resulting in the distortion of a transmission signal.
These channel characteristics have different sizes and phases according to time and path. In order to obtain an original transmission signal from a received signal, channel state information of the transmission signal, which is related to the signal distortion, needs to be known. As well known to those having ordinary skill in the art, a training symbol that is previously defined between a transmitter and a receiver is necessary in order to estimate a channel in a mobile station. In particular, training symbols that can be used in the downlink of systems to which the IEEE 802.16e standard is applied, or a Wibro system of the systems include a preamble and a pilot.
The preamble is transmitted through a first OFDMA symbol of the entire downlink frame, and the pilot is transmitted through the entire OFDMA symbols of a downlink frame except for the preamble. Therefore, in a communication mobile station, a channel has to be estimated by employing the preamble and/or the pilot, and an original transmission signal has to be acquired from a received signal by employing the estimated channel.